Water and wastewater are commonly treated using a variety of techniques. In one currently known system 10 and method, as shown in FIG. 1, wastewater is initially treated in a bioreactor 12 and is then transferred to a basin 14 comprising a partial mix aerated cell 16 and a polishing cell 18. As depicted in FIG. 1, the reactor 12, partial mix cell 16 and polishing cell 18 are provided in series. The wastewater flows from the reactor 12 to the partial mix cell 16 and then from the partial mix cell 16 to the polishing cell 18.
In this prior system 10, the reactor 12 may be utilized to achieve nitrification, denitrification, removal of carbonaceous biochemical oxygen demand (BOD), and/or removal of ammonia. During the treatment of the wastewater in the reactor 12, sludge, such as waste activated sludge (WAS), and excess biosolids are generated. The sludge and excess biosolids generated in the reactor 12 flow with the wastewater into the partial mix cell 16. The sludge and biosolids settle, accumulate and form a benthal layer within the partial mix cell 16.
During warm weather, the sludge and biosolids are digested in the reactor 12 and/or partial mix cell 16, and are stabilized. When the temperature of the wastewater is at or above a certain temperature, which may be in a range between about 12° C. and 18° C. (e.g., at or above about 15° C.), the sludge and biosolids are anaerobic and aerobically digested in a generally complete reaction and therefore no ammonia is released from the sludge into the effluent flow discharged from the system 10. The digested sludge and biosolids are stable even when the temperate drops.
During cold weather, the sludge and biosolids accumulate, settle into the benthal layer and are essentially refrigerated and stored. There is typically little breakdown, digestion or biological degradation of the settled sludge and biosolids either in the partial mix cell 16 or the polishing cell 18 once the temperature of the wastewater drops below a certain temperature, which may be in a range between about 12° C. and 18° C. (e.g., below 15° C.). Once the temperature of the wastewater drops, digestion generally ceases, even though air is still being applied in the partial mix cell 16. Even though the sludge reaction is stopped, the wastewater exiting the partial mix cell 16 is still generally of high quality due to the treatment it has undergone in the reactor 12 and the solids settling in the partial mix cell 16. Any small amount of solids that are discharged with the wastewater from the partial mix cell 16 can be captured in the polishing cell 18 and the effluent flow discharged from the system 10 is generally of high quality.
Accordingly, the operating conditions within the system 10, and the treatment of the wastewater therein, are generally satisfactory during the warm weather months and cold weather months. However, as described below, the conditions and treatment during the spring season become temporarily ineffective and unacceptable.
During the spring season, the temperature of the wastewater begins to increase. Once the temperature of the sludge and biosolids within the partial mix cell 16 or polishing cell 18 increases to a certain temperature, which may be in a range between about 12° C. and 18° C. (e.g., at or above about 15° C.) or higher for an extended period of time, digestion of the stored solids resumes. As set forth above, all of the sludge and biosolids generated by the reactor 12 during the cold weather months are stored within the partial mix cell 16 or the polishing cell 18 during the cold weather months and are not digested. The length of time that the cold weather persists influences the amount of sludge and biosolids that accumulate in the partial mix cell 16 or the polishing cell 18 and remain undigested.
Once digestion resumes during the warmup in the spring season, the digestion of the sludge and biosolids increases at a logarithmic rate, thereby placing a large load on the partial mix cell 16 and polishing cell 18 for a relatively short period of time (e.g., from about two to six weeks). This spike in digestion is a predictable event that occurs each spring as the temperature of the wastewater within the system 10 rises, particularly in systems 10 located in northern regions. The rapid increase in digestion and biological activity releases ammonia from the stored biosolids into the partial mix cell 16 and polishing cell 18 thereby resulting in an effluent flow having an elevated ammonia concentration. Wastewater from the partial mix cell 16 having elevated ammonia concentrations passes through the polishing cell 18 and is discharged from the system 10 in the effluent flow. The ammonia concentrations during this period of time are very often above the levels permitted by regulatory agencies. Regulatory agencies, such as the Environmental Protection Agency (EPA), have implemented restrictions on the amount of ammonia that may be contained within effluent water discharged into streams, rivers and other bodies of water. The rebound in the amount of ammonia resulting from the rapid digestion of the sludge and biosolids that have been stored during the winter creates an ammonia level in the effluent flow that typically exceeds the limits set by the EPA and/or similar state and local agencies, particularly in systems 10 located in northern regions. This phenomenon is often referred to as “ammonia rebound” or “biological rebound.” Again, the amount of sludge and biosolids produced in the reactor 12 over the winter months greatly influences the magnitude of the ammonia rebound.
Accordingly, a need exists for a wastewater treatment system and method adapted for preventing or reducing ammonia rebound or biological rebound during periods of warming weather. A need also exists for a method of modifying an existing wastewater treatment system such that it is adapted for operating in a manner to prevent or reduce such biological or ammonia rebound.